Interview Questions - Microsoft - What is the state of each door after the last pass ?

There are 100 doors in a row that are all initially closed. You make 100 passes by the doors starting with the first door every time. The first time through you visit every door and toggle the door (if the door is closed, you open it, if its open, you close it). The second time you only visit every 2nd door (door #2, #4, #6). the third time, every 3rd door (door #3, #6, #9), etc, until you only visit the 100th door. What is the state of each door after the last pass?
I think ,this is one of 5 most frequently asked puzzles in the interviews.